1. Field of the Invention
The present invention relates to a method for producing a polarizing plate. The present invention also relates to a polarizing plate obtained by the method. The polarizing plate may be used alone or in the form of a laminated optical film to form an image display such as a liquid crystal display (LCD), an organic electro-luminescent (EL) display, a cathode ray tube (CRT), or a plasma display panel (PDP).
2. Description of the Related Art
A variety of LCDs for use in televisions, personal computers, cellular phones, and so on are based on liquid crystal switching to visualize the polarization state, and based on the display principle, they use polarizers. Iodine polarizers having a structure produced by adsorbing a dichroic material such as iodine to a hydrophilic polymer film such as a polyvinyl alcohol film and stretching the film have high transmittance and optical properties of high degree of polarization. Therefore, they are the most popular and widely used polarizers. Specifically, in the process of uniaxially stretching a polyvinyl alcohol film, a dichroic material (as a dye) adsorbed to the polyvinyl alcohol molecule is oriented to constitute a polarizer. A method of stretching a polyvinyl alcohol film to a high stretch ratio is proposed to highly orient a dichroic material and to improve the optical properties (JP A 2002-28971). On the other hand, the polarizer should be protected from the external environment and prevented from expansion and shrinkage associated with heating/cooling or moisture absorption/release. In order to do so, therefore, the polarizer is used in the form of a polarizing plate, which includes the polarizer and a transparent protective film bonded to one side or both sides of the polarizer with an adhesive layer interposed therebetween.
As mentioned above, polarizing plates are used for a variety of LCDs. With a reduction in the thickness of small-sized LCDs for cellular phones or the like, a reduction in the thickness of polarizing plates has become an urgent need. In order to reduce the thickness of a polarizing plate, the thickness of its polarizer or its transparent protective film may be reduced.
The thickness of a polarizer can be reduced by reducing the thickness of a polyvinyl alcohol film, a raw material for the polarizer. However, if a thin polyvinyl alcohol film is used as a raw film and stretched to a high stretch ratio so that high optical properties can be obtained in the process of producing a polarizer, the film may be ruptured so that the productivity may be seriously reduced. On the other hand, if the stretch ratio is set at a low level, a problem may occur in which satisfactory polarizing properties cannot be obtained.
Specifically, when a polarizer of a polyvinyl alcohol film is uniaxially stretched to a high stretch ratio in the longitudinal direction, its width and thickness are reduced depending on the stretch ratio. In order to achieve high polarizing properties in this case, the polarizer should generally be stretched to a higher stretch ratio. Therefore, the raw film for the polarizer should be somewhat thick so that a high-performance uniform polarizer having satisfactory swelling properties and stretchability can be obtained. Since film stretching is based on permanent deformation caused by application of stress and impress to the film, weaker regions of the film are more intensively stretched. Therefore, highly stretched portions of a film are more reduced in thickness and more likely to be a cause of film rupture. This is more significant when the raw film is thinner or stretched to a higher stretch ratio. Therefore, it has been difficult to use a thin raw film, because it is necessary to reduce film ruptures as much as possible for stable polarizer production.
On the other hand, there is a problem in which when a thin transparent protective film is used, the resulting polarizing plate may be bent because of its reduced stiffness. In particular, the shrinkage of a polarizer in an absorption axis direction in a thermostatic humidified environment, which is considered to be based on relaxation of polarizer stretching, becomes a cause of liquid crystal panel warp, when a polarizing plate is bonded in the LCD manufacturing process. Under certain circumstances, the resulting LCD may be significantly bent and come into contact with a backlight or a backside film such as a diffusing plate so that the display characteristics may be significantly degraded, or the resulting LCD may come into contact with a bezel for fixing the liquid crystal panel to cause the problem of light leakage.
In recent years, the size of polarizing plates for use in LDCs has been necessarily increased with the greater size of the LCDs, but the thickness of liquid crystal panels themselves has not been changed. Therefore, there is a problem in which distortion associated with the expansion and shrinkage of polarizing plates may cause bending of LCDs themselves. This may cause major trouble with the display characteristics, and therefore a reduction of the shrinkage of polarizing plates has been demanded. More specifically, heating- or humidifying-induced relaxation of polarizers may cause the phenomenon of significant shrinkage in the stretching direction, which may cause the problem of the generation of stress or distortion.
For the problems described above, rigid transparent protective films may be used, or thinner polyvinyl alcohol films may be used as described above. In the former case, however, the stress generated on expandable and shrinkable polyvinyl alcohol films themselves cannot be reduced, and in the latter case, there are limits to the reduction in the thickness of polyvinyl alcohol films as described above. In addition, a polyvinyl alcohol film may be stretched to a low stretch ratio so as to form a polarizer with relaxed orientation and reduced contractile force. However, if the stretch ratio is lowered so that the contractile force can be reduced, the iodine complex orientation itself will be reduced, so that serious degradation in the optical properties may occur and that stretching uniformity may be reduced, which makes it difficult to produce a polarizing plate with good properties.